1//===----------------------------------------------------------------------===//
 2//
 3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 4// See https://llvm.org/LICENSE.txt for license information.
 5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 6//
 7//===----------------------------------------------------------------------===//
 8
 9#include <__config>
10#include <__mutex/once_flag.h>
11#include <__utility/exception_guard.h>
12
13#if _LIBCPP_HAS_THREADS
14#  include <__thread/support.h>
15#endif
16
17#include "include/atomic_support.h"
18
19_LIBCPP_BEGIN_NAMESPACE_STD
20
21// If dispatch_once_f ever handles C++ exceptions, and if one can get to it
22// without illegal macros (unexpected macros not beginning with _UpperCase or
23// __lowercase), and if it stops spinning waiting threads, then call_once should
24// call into dispatch_once_f instead of here. Relevant radar this code needs to
25// keep in sync with:  7741191.
26
27#if _LIBCPP_HAS_THREADS
28static constinit __libcpp_mutex_t mut  = _LIBCPP_MUTEX_INITIALIZER;
29static constinit __libcpp_condvar_t cv = _LIBCPP_CONDVAR_INITIALIZER;
30#endif
31
32void __call_once(volatile once_flag::_State_type& flag, void* arg, void (*func)(void*)) {
33#if !_LIBCPP_HAS_THREADS
34
35  if (flag == once_flag::_Unset) {
36    auto guard = std::__make_exception_guard([&flag] { flag = once_flag::_Unset; });
37    flag       = once_flag::_Pending;
38    func(arg);
39    flag = once_flag::_Complete;
40    guard.__complete();
41  }
42
43#else // !_LIBCPP_HAS_THREADS
44
45  __libcpp_mutex_lock(&mut);
46  while (flag == once_flag::_Pending)
47    __libcpp_condvar_wait(&cv, &mut);
48  if (flag == once_flag::_Unset) {
49    auto guard = std::__make_exception_guard([&flag] {
50      __libcpp_mutex_lock(&mut);
51      __libcpp_relaxed_store(&flag, once_flag::_Unset);
52      __libcpp_mutex_unlock(&mut);
53      __libcpp_condvar_broadcast(&cv);
54    });
55
56    __libcpp_relaxed_store(&flag, once_flag::_Pending);
57    __libcpp_mutex_unlock(&mut);
58    func(arg);
59    __libcpp_mutex_lock(&mut);
60    __libcpp_atomic_store(&flag, once_flag::_Complete, _AO_Release);
61    __libcpp_mutex_unlock(&mut);
62    __libcpp_condvar_broadcast(&cv);
63    guard.__complete();
64  } else {
65    __libcpp_mutex_unlock(&mut);
66  }
67
68#endif // !_LIBCPP_HAS_THREADS
69}
70
71_LIBCPP_END_NAMESPACE_STD